Acidified milk drinks are becoming increasingly popular amongst consumers. Such drinks have long been commercially successful in Japan and parts of south east Asia and are now being introduced into western markets. Such drinks may be yoghurt-based (in which case they are often called drinking yoghurts), Lactobacillus drinks or soft drinks based upon milks These drinks differ from each other for instance in their respective contents of milk solids non-fat (MSNF). MSNF is principally casein. Yoghurt drinks typically contain a minimum of 8% by weight of MSNF, Lactobacillus drinks contain a minimum of 3% by weight of MSNF, whereas soft drinks contain less than 3% by weight of MSNF.
Drinking yoghurts are either distributed fresh and promoted for their content of live lactobacilli, or heat treated prior to distribution to obtain extended shelf life. Heat treated yoghurt drinks must be stabilised to prevent sedimentation of casein particles as such sedimentation leads to the drinks developing an undesirable sandy mouth feel. Even low viscosity MSNF fresh acidified milk drinks have to be stabilised to prevent precipitation of casein particles.
Pectin is most commonly used as the stabiliser for acidified milk drinks. Pectin is a structural polysaccharide commonly found in the form of protopectin in plant cells. The backbone of pectin comprises .alpha.-1-4 linked galacturonic acid residues which are interrupted with a small number of 1,2 linked .alpha.-L-rhamnose units. In addition, pectin comprises highly branched regions with an almost alternating rhamno-galacturonan chain. These highly branched regions also contain other sugar units attached by glycosidic linkages to the rhamnose or galacturonic acid units. The long chains of .alpha.-1-4 linked galacturonic acid residues are commonly referred to as "smooth" regions, whereas the highly branched regions are commonly referred to as "hairy" regions.
Some of the carboxyl groups of the galacturonic residues are esterified, typically by methyl groups. The remainder are present as free carboxyl groups. Esterification of the carboxyl groups occurs after polymerisation of the galacturonic acid residues. However, it is extremely rare for all of the carboxyl groups to be esterified. Usually, the degree of esterification varies from 0 to 90% of the available carboxylic groups. If 50% or more of the carboxyl groups of a pectin are esterified, then the pectin is commonly referred to as being a high ester pectin or high methoxyl pectin. If less than 50% of the carboxyl groups are esterified, then the pectin is commonly referred to as being a low ester pectin or a low methoxyl pectin. If the pectin does not contain any, or only a few, esterified groups, it is commonly referred to as pectic acid.
The structure of the pectin, in particular the degree of esterification, dictates many of its physical and/or chemical properties. For example, pectin gelation caused by the presence of calcium cations depends especially on the degree of esterification. Gelation is believed to result from the calcium ions forming cross-linked complexes with free carboxyl groups of a number of pectin chains causing the formation of a continuous three-dimensional gelled matrix.
It is known that the distribution of the free carboxyl groups along the polymer chain is important for determining whether the pectin is suitable for use as a stabiliser for acidified milk drinks. It has been proposed that pectin stabilises a suspension of casein particles by adsorbing onto the surface of the casein particles at specific points of the pectin molecule. The remainder of the pectin molecule forms dangling chains and loops that protrude into the liquid phase. The repulsion between the resulting complexed particles may be due to the increased osmotic pressure created when pectin chains complexed to two casein particles interact with one another.
For a pectin to be useful as a stabiliser of an acidified milk drink at least some of its free carboxyl groups have to be arranged in blocks (i.e. contiguously) rather than being randomly distributed discretely along the polymer chain. The binding force between a pectin molecule and a casein particle is due to the blocks of negatively charged carboxyl groups interacting with positive charges which exist on the particle surface. The length of the blocks of free carboxyl groups is also important. Blocks of carboxyl groups that are either too long or too short do not result in stabilization of the system. In the former case, no dangling chains exist. In the latter case, the pectins do not securely attach themselves to the casein particles and therefore do not lead to stabilisation of the particles.
A well-known characteristic of low ester or calcium sensitive pectin is its ability to thicken or form gels, particularly when in the presence of alkaline earth metal cations such as Ca++. Unfortunately, acidified milk drinks naturally contain significant quantities of calcium cations. These cations have the undesirable effect of causing significant viscosity increases if excess pectin is present which can even result, in extreme cases, in the acidified milk drink gelling.
Production of heat-treated whey drinks also presents problems. Heat treatments above 70.degree. C. cause variable amounts of whey particles to form depending upon the precise temperature reached. The amount of pectin necessary to stabilise the heat-treated whey drink therefore varies with the heat treatment. As the whey particles that are formed are relatively small, the amount of pectin needed to obtain a stable drink can be very high due to the large total surface area of these particles. However, excessive addition of pectin will again result in thickening or gel formation due to cross-linking of the excess pectin with the naturally present calcium cations.
It will therefore be understood that the inclusion of pectin has both desirable and undesirable effects on the properties of acidified milk drinks. Whilst it can act as a stabiliser against sedimentation of casein particles or whey separation, it can have the disadvantage of increasing the viscosity of the drink due to its cross-linking with naturally co-present calcium cations rendering the drink unpalatable. It will be seen that in the absence of pectin, there is significant sedimentation in the case of both drinks caused by the instability of the casein particles which also results in relatively high viscosity. After a certain concentration of pectin has been added, the casein particles become stabilised against sedimentation after which increasing the pectin concentration has little effect on sedimentation. Turning to the viscosity of the drinks, this also significantly drops on stabilisation of the casein particles but then almost immediately begins to rise again due to cross-linking of the excess pectin added by the co-present calcium cations. This increased viscosity is undesirable as it leads to the beverage having poor organoleptic properties. This range may be as narrow as only 0.06% by weight of pectin based upon the beverage weight as a whole. Below this working range, sedimentation is a significant problem, whereas above it, the viscosity of the beverage is undesirably high.
Commercially, it is critical for manufacturers of acidified milk drinks to avoid sedimentation as this would ruin the drink. Therefore manufacturers typically add excess pectin to ensure that sedimentation does not occur but the excess pectin which is added results in the drinks having an undesirably high viscosity. Whilst manufacturers of course would like to target the narrow working range previously mentioned, this is commercially difficult due to the risk that insufficient pectin may be added causing the whole batch of drink to fail due to sedimentation.
It is well known that the methyl content of pectin is modified in nature by plant pectin esterases which are present in the plant tissue. These esterases, usually called pectin methyl esterases (PMEs), demethylate esterified carboxylic groups which are next to at least two contiguous free carboxylic acid groups. The demethylation proceeds in this way forming blocks. As previously mentioned, it is the arrangement of these blocks which is important for the stabilising action of pectin in acidified milk drinks. The proteases papain and bromelain are also known to demethylate pectins.
In a commercial, extracted pectin having a typical degree of esterification of approximately 70-74%, the length of the free carboxylic blocks may vary from molecule to molecule, and each pectin molecule typically includes several blocks of different lengths. The binding force between the pectin molecule and the surface of the casein particle depends on the length of the blocks which interact with the positive charges on the casein surface and/or with negative charges on the casein surface via calcium salt bridges. To obtain complete stability a significant proportion of the surface of a casein particle should be covered by the pectin.
As described in EP-A-0 664 300, pectins extracted from typical sources such as citrus peel can be separated or fractionated into two distinct pectin fractions. One fraction is a calcium sensitive pectin (CSP) and the other is a non-calcium sensitive pectin (NCSP). These two pectin fractions possess quite different calcium sensitivities (.DELTA.CS). CSP is a high ester pectin having a degree of esterification of at least 65% whose carboxyl groups are arranged predominantly in blocks. The block configuration of the carboxyl groups is responsible for CSP's calcium sensitivity. On the other hand, NCSP is also a high ester pectin having a degree of esterification of at least 70% whose free carboxyl groups are randomly arranged along the polymer chain with no significant degree of contiguity.
Typically commercial pectins which are marketed for use in the stabilisation of acidified milk drinks contain approximately 60% of CSP and 40% of NCSP. The pectin as a whole may have a .DELTA.CS value (as defined below) of 200 to 600 cP, whereas the CSP fraction usually has a .DELTA.CS value of 500 to 1500 cP. Since it is the CSP fraction which is responsible for stabilising casein in acidified milk drinks, the very high calcium sensitivity of this fraction is detrimental to the viscosity of the drink when the pectin, and therefore also its CSP fraction, is present in excess. In addition, since the NCSP fraction does not significantly contribute to the stabilising power of the pectin as a whole, its presence is wasted. Naturally, it would be highly desirable to have all of the pectin which is added to the acidified milk drink contribute to the stabilisation of the casein and for that pectin to have its calcium sensitivity controlled so that it does not significantly increase the viscosity of the drink when present in excess.